1. Field of the Invention
This invention relates to a process for the stabilization of certain fluoropolymers, especially those useful in many high technology applications.
2. Background
Many fluoropolymers are known in the art. They include especially various copolymers of two or more comonomers such as, for example, tetrafluoroethylene (TFE), hexafluoropropene (HFP), chlorotrifluoroethylene (CTFE), perfluoro(methyl vinyl ether) (PMVE), perfluoro(propyl vinyl ether) (PPVE), and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD).
Such copolymers may be melt-processible and thus can be fabricated at high temperatures. However, they frequently suffer some deterioration during high temperature processing and thus lose some of their desirable properties such as, for example, freedom from bubbles. This thermal deterioration can be traced to the presence of various labile end groups, such as, e.g., carboxyl (--COOH) and fluorocarbonyl (--COF). The former tends to eliminate carbon dioxide at high temperatures, while the latter, while more thermally stable, nevertheless tends to hydrolyze in the presence of moisture, which normally cannot be completely avoided, and is converted to carboxyl groups. Such hydrolysis also results in the evolution of hydrofluoric acid which is corrosive to most materials of industrial importance, including many metals, glass and quartz.
The removal of unstable end groups has long been an important part of the technology of perfluorinated melt-processible copolymers of TFE. Schreyer, U.S. Pat. No. 3,085,083, discloses the treatment of such polymers "with water, preferably in the presence of inorganic compounds having a pH of at least 7, such as stable bases, . . . at a temperature of 200.degree.-400.degree. C., and recovering a fluorocarbon polymer having at least half of all the end-groups in the form of difluoromethyl groups" (--CF.sub.2 H). There is no suggestion in this patent, which discloses the use of inorganic treating agents, of the unexpected results achieved by means of this invention which requires the use of secondary or tertiary amines.
Buckmaster et al., U.S. Pat. No. 4,675,380, disclose the fluorination of melt-processible TFE copolymers which have been coagulated by stirring in the presence of a mineral acid and a water-immiscible liquid and then isolated. The total number of unstable end groups was reduced to less than 80 per 10.sup.6 carbon atoms.
U.K. Patent 1,210,794 to Du Pont discloses the fluorination of fluorocarbon copolymers to reduce the number of unstable end groups. The process of that patent, when carried out with some of the copolymers of interest in the present invention, needs to employ a fluorination temperature of at least 225.degree. C. to remove unstable end groups.
Although fluorination of fluoropolymers can be employed to reduce the concentration of multiple bonds and unstable end groups, complete fluorination requires high temperatures, usually above 200.degree. C., to remove substantially all --COF groups. However, if the polymers soften or begin melting at the fluorination temperature, such a process causes agglomeration of polymer particles, which leads to difficulties in their further handling and processing.
Furthermore, high temperature fluorination can cause equipment corrosion, and it is difficult to handle fluorine safely because it is toxic and is a strong oxidizing agent. It would be desirable to be able to remove --COOH and --COF groups from fluoropolymers without the use of fluorine, and preferably without having to heat the polymer above its melting point.
Carbon- and graphite-filled fluorocarbon compositions for electrical applications have been known for some time. They are primarily used in preference to other conductive polymers when chemically active and/or high temperature environments are to be encountered. Applications in which conductive fluorocarbons containing carbon black and/or graphite are used include current-limiting devices, e.g., self-regulating heater cable (U.S. Pat. Nos. 4,318,881, 4,624,990 and 4,545,926), antistatic containers for semiconductor chip processing (J6 1027842A), conductive coating compositions (U.S. Pat. Nos. 4,482,476, 4,064,074 and EP 79589), battery electrode constructions (U.S. Pat. Nos. 4,468,362, 3,676,222, EP 126511A and JA 7016669 R), thermally and electrically conductive caulk (U.S. Pat. No. 4,157,327), antistatic/antifriction sheets for use in tape or film cassettes (U.S. Pat. No. 3,908,570) and conductive filaments (J 58163725A and J 75013957).
However, there are difficulties associated with adding carbon black to fluoropolymers to achieve conductivity. One difficulty is the relatively large and rapid rise that occurs in effective melt viscosity of the blend as carbon black is added. This large and rapid viscosity increase results in more difficult and time consuming processing. At low enough levels of carbon black to be of little influence on effective melt viscosity, the electrical conductivity is usually lost or in a range below that desired. Any means to reduce melt viscosity and/or reduce the carbon black concentration to lower levels, while maintaining the desired conductivity, is a very desirable goal.